The present invention generally relates to a method of manufacture for a magnetic recording media layer, and an apparatus therefor, specifically a method for orienting magnetic pigment particles contained in a magnetic recording layer for optimum signal to noise ratio.
Formulations to be coated onto substrates for use as magnetic recording layers or lower support layers are required to be extremely thin coatings. Such formulations are typically magnetic or other pigment particles dispersed in binder systems with various adjuvants, which are custom and proprietary to magnetic recording manufacturers, and which vary with the type and variation of magnetic media produced. Many, if not most, coating formulations are coated onto plastic substrates of one type or another.
Conventionally, manufacture of magnetic recording media has included the process of orienting magnetic particles in the magnetic layer(s) in order to improve the recording density. Various processes for making recording media have oriented the magnetic pigment particles in a direction parallel to the surface of the magnetic layer and in the longitudinal direction of the magnetic recording medium; other processes have oriented in oblique and transverse directions. The recommended method orients the magnetic pigments into the preferred direction of the given medium format consistent with tracks and head designs.
After the magnetic layer has been formed upon the substrate, or upon a first non-magnetic layer coated on the substrate, the particles in the layer are typically oriented during the drying process. Placement of the magnets is an important issue in the orientation of the magnetic layer, especially with the very thin magnetic layers now preferred for increased recording density. When magnets are placed late in the drying process, the thinner layers have dried quickly because of the small amount of solvent present, and the subsequent orienting operations result in only minimal improvement to tape performance. However, attempts to place magnets earlier in the drying cycle have been unsuccessful. Early placement of magnets, at a point where the viscosity of the coating is still very low and the particles are still mobile, has typically caused the magnetic particles to clump and form long chains throughout the magnetic layer. This is highly detrimental to the recording density and quality of the resulting medium.
It has now been discovered that a magnetic recording medium exposed to a magnetic field e.g., up to about 2500 gauss created by permanent magnets or electric coils, at a point when the average percent solids of the multiple layers is less than 40% collectively, and exposing the magnetic recording medium to at least one additional magnetic field of from about 3500 to about 7000 gauss after partial drying when the average percent solids of said multiple layers is from about 43% to about 60% collectively exhibits significantly improved skirt signal-to-noise ratios over a magnetic recording medium only exposed to the higher strength magnetic field(s).
The invention provides a method for manufacturing a magnetic recording medium which includes the step of orienting the magnetic coating to provide magnetic squareness and orientation ratio.
Specifically, the invention provides a method of manufacturing a magnetic recording medium comprising the steps of:
a) forming at least one support layer selected from a non-magnetic or soft magnetic layer on a non-magnetic substrate;
b) forming at least one magnetic layer comprising magnetic pigment particles atop said at least one support layer to form a magnetic recording medium having multiple layers;
c) exposing said magnetic recording medium to a first magnetic field of up to about 2500 gauss when the average percent solids of said multiple layers is less than 40% collectively, and
d) exposing said magnetic recording medium to at least one additional magnetic field of from about 3500 to about 7000 gauss when the average percent solids of said multiple layers is from about 43% to about 60% collectively, wherein said magnetic pigment particles in said magnetic layer are oriented in a longitudinal direction.
In another aspect of the invention, a magnetic recording medium is provided which exhibits a significant increase in skirt signal-to-noise ratio, such medium comprising a substrate having coated at least one magnetic layer comprising magnetic pigment particles, said magnetic pigment particles having been oriented longitudinally by means of exposure to a first magnetic field of up to about 2500 gauss when the multiple layers have an average percent solids of less than 40% collectively, and exposure to at least one additional magnetic field of from about 3500 to about 7000 gauss when the multiple layers have an average percent solids of from about 43% to about 60% collectively, wherein this magnetic recording medium exhibits at least a 0.5 decibel (dB) increase over an identical magnetic recording medium not exposed to said 2500 gauss magnetic field when said average percent solids of said multiple layers collectively is less than 40%.
In another aspect of the invention, a layer magnetic recording medium is provided which exhibits a significant increase in skirt signal-to-noise ratio, such medium comprising a substrate having coated thereon at least one support layer, and at least one magnetic layer comprising magnetic pigment particles, the magnetic pigment particles having been oriented longitudinally by means of exposure to a first magnetic field of up to about 2500 gauss when the multiple layers have an average percent solids of less than 40% collectively, and exposure to at least one magnetic field of from about 3500 to about 7000 gauss when the multiple layers have an average percent solids of from about 43% to about 60% collectively. This magnetic recording medium exhibits an increase in skirt signal-to-noise ratio of at least about 1 decibel over an identical magnetic recording medium not exposed to said 2500 gauss magnetic field when said average percent solids of said multiple layers collectively is less than 40%. As used herein, these terms have the following meanings:
1. The term xe2x80x9cwebxe2x80x9d refers to a support for transporting the magnetic recording media through the manufacturing process.
2. The term xe2x80x9ccoercivityxe2x80x9d means the intensity of the magnetic field needed to reduce the magnetization of a ferromagnetic material to zero after it has reached saturation.
3. The term xe2x80x9cOerstedxe2x80x9d, abbreviated as Oe, refers to a unit of magnetic field in a dielectric material equal to 1/xcexc Gauss, where xcexc is the magnetic permeability.
4. The terms xe2x80x9clayerxe2x80x9d and xe2x80x9ccoatingxe2x80x9d are used interchangeably to refer to a coated composition.
5. The term xe2x80x9cskirt noisexe2x80x9d means the noise in a measurement bandwidth of about 2 MHz centered about the signal frequency used, when tested according to ECMA International Standard 319.
6. The term xe2x80x9cbroadband signal-to-noise ratioxe2x80x9d, usually abbreviated xe2x80x9cBBSNxe2x80x9d, is the ratio of average signal power to average integrated broad noise power of a tape clearly written at density TRD2, and is expressed in decibels (dB).
5. The term xe2x80x9ccalenderingxe2x80x9d refers to passing material through heated cylinders or rolls for such purposes as to render the thickness uniform, to increase surface gloss, or to force a top layer to impregnate a lower layer.
6. The term xe2x80x9cin-linexe2x80x9d means occurring in the same manufacturing line as the majority of the other processes for manufacture of the magnetic recording medium.
7. The term xe2x80x9caverage percent solids collectivelyxe2x80x9d means the average of the percent solids of all of the multiple layers coated onto the front side of the magnetic recording medium, both support layers and magnetic layers.
All portions, percents and ratios herein are by weight unless otherwise specifically stated.